Still trim: The Duchess of Cambridge in her short pastel coat on Sunday

While her stylish mint-green Mulberry coat, glittering earrings, cappuccino fascinator and those ubiquitous nude heels gave fans of the Duchess of Cambridge plenty to admire when she stepped out this week, it wasn’t just Kate’s outfit that drew attention — it was her bump.

Or lack of it.

The much-anticipated royal baby is due in July, making the Duchess six months pregnant, but she is only just beginning to show.

The pictures have prompted comparisons with other famous mums-to-be at the same stage, including her late mother-in-law, Princess Diana, Victoria Beckham and actress Kate Hudson, all of whom were far larger than Kate.

According to experts, Kate’s tiny bump is by no means a cause for concern, and could be the result of anything from her slim, muscular physique to the position of the baby in the womb.

Simon Mehigan, consultant midwife at Liverpool Women’s Hospital, explains that smaller bumps are common in first pregnancies and are more an indicator of the woman’s body shape than anything about the baby itself.

‘It’s Kate’s first baby, so she’s going to have good abdominal muscles, meaning everything will be held in nice and tightly,’ he says.

‘It’s not unusual for women to not show very much during their first pregnancy.

‘But it can be a different story with your second and third babies.

'No matter how many sit-ups you do, everything’s been stretched and it won’t go back to how it was.’

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Clearly showing: Holly Willoughby on the red carpet with her second child

Malcolm Dixon, obstetrician and gynaecologist at Rochdale Infirmary, agrees. ‘If it’s a first baby, the tummy muscles tend to be taut and strong, so the baby is held in tightly. This is especially true of women who are sporty and do their sit-ups, and we know that Kate’s an athletic soul.

‘She’s also quite a slender girl and they tend to hold bumps well, too. If you’ve already got a bit of fat around the stomach, a small baby bump might appear larger, whereas if you’re slim there’s nothing to show except the baby. It makes no difference to how big the baby will be when it arrives — it’s just how it looks.’

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Sometimes, however, the position in which the baby is lying can affect how a bump appears.Mr Dixon says: ‘If the baby has its back at the front, its arms and legs are tucked away, making it appear neater, whereas if the back is lying next to the mother’s back, its arms and legs will be sticking out.

‘It’s something I have seen in my clinics.’

Body shape: Kate Hudson had a huge baby bump despite her tiny frame

Flaunting it: Minnie Driver in a tight black dress at a premiere

The Duchess’s severe morning sickness, which saw her admitted to hospital in December, will not have had any bearing on the weight of her baby, says Mr Mehigan.

‘With hyperemesis gravidarum [the condition Kate suffered] a baby will still grow normally, because it will take all the goodness it needs. The mother will feel rotten, but the baby should be perfectly healthy.’

At 5ft 10in, the Duchess’s height, with her perfect posture, may also explain her subtle pregnancy bump. ‘Tall women, especially those with a longer body, tend not to look so big when pregnant because the baby’s not crammed into a small space,’ says Gail Johnson, professional adviser at the Royal College Of Midwives.

‘And if you stand nice and straight and upright, you tend to pull in your stomach muscles.’

While there’s no evidence that genes play a role in how big your bump is, Ms Johnson says some women may find they follow a similar pattern to their mother, because they have a similar body shape and lifestyle.

‘However, that doesn’t necessarily mean that if you’re tall it will be a more comfortable pregnancy — if the baby is more tightly packed in, it may be pushing on your bladder.’

The right dress can fix a multitude of figure issues. Kate’s carefully chosen maternity wardrobe has no doubt flattered her changing shape — and she’s been helped by the chilly weather we’ve had. ‘As Kate’s pregnancy began during the winter months, she’s been concealed in beautiful tailored coats, so it’s not surprising we haven’t noticed a bump,’ says Michelle Lee, founder and director of Keungzai, a fashion label specialising in maternity wear.

Famous curves: Kim Kardashian in Los Angeles this month

Petite: Myleene Klass had a very pronounced bump with her second child

‘Some women like to wear more figure-hugging clothes when pregnant, but Kate’s maintained her style, which has always been chic and classic.’

The one thing on which all midwives agree is that no two bumps are the same, and you can tell very little about a pregnancy just by looking.

‘The pregnancy will follow a set pattern that is normal and right for the woman and her baby,’ says midwife Gail Johnson.

Expecting William: Princess Diana in March 1982

‘It might be that there’ll be a growth spurt and she’ll start to show very quickly towards the end, but some women never do and it’s just the way it is. It’s not about catching up.’

Obstetrician Malcolm Dixon agrees: ‘Being petite allows you to get away with having a less big bump, but you can’t predict anything from looking. It’s an inexact science — you need to do something like an ultrasound or feel the bump with your hands.’

He adds that the fact Kate’s hyperemesis meant she was forced to announce her pregnancy to the public before she had reached the three-month stage means we have known about it for longer and so may feel like she should look bigger than she is.

And what about the folklore that the size and shape of the bump can predict the gender of the baby?‘Utter nonsense,’ says Gail Johnson. The evidence bears this out. A U.S. study of 104 pregnant women in the journal Birth found that the shape and size of the mothers’ stomachs were not related to the gender of their baby.

To the researchers’ surprise, a far more accurate predictor were the mother’s dreams and intuition about the babies’ sex.

So while Kate might have already dreamed of her Prince or Princess, the rest of us will just have to wait and see.